During the operation of rotary machinery, it is essential that the rotating elements of the machine be in dynamic balance thereby limiting unwanted vibrational forces. Because these vibrational forces are proportional to the mass and square of the speed of rotation, precautions should be taken to insure dynamic balance and prevent excessive vibration which can result in undue wear and premature breakdown and failure.
The present invention has general utility and application to a wide variety of machinery wherein vibration testing is important; however, its structure and function are best appreciated when related to an aircraft. In the aircraft industry, engines are employed which have massive moving parts or rotors in the form of fans or turbines that rotate at relatively high angular velocities. Engine failure due to rotor imbalance can result in serious damage to the aircraft as well as its occupants and therefore, it is desirable to monitor the operation of these engine rotors in order to rectify an out-of-balance condition prior to failure.
U.S. Pat. No. 4,010,637, assigned to the Lockheed Corporation, assignee of the present invention, discloses a system for monitoring engine vibrations utilizing an uniform level filter to condition the output signal from an accelerometer vibration transducer mechanically coupled to the engine. The filter is adjusted in advance to receive different engine rotor frequencies which are then modulated or combined into a single output relative to any of several g-forces of interest. Thus, a single limit output is established and preset which if exceeded alerts an operator to an out-of-balance condition.
The arrangement noted above is operationally quite efficient, but does possess certain definite disadvantages. The arrangement fails to determine the degree of imbalance of the defective rotor and the specific location of the weight to be added to that rotor to compensate for such imbalance. As a result, the balancing techniques that are used to rectify the out-of-balance condition are trial and error in nature, requiring several test runs. This approach is not only time consuming and expensive, but also contributes to unwanted wear and tear on the engine due to operation at high vibration levels.
A method has been developed and is disclosed in U.S. Pat. No. 4,238,960, assigned to Lockheed Corporation, which reduces maintenance cost and downtime for rotor balancing by utilizing inflight data to determine corrective weight magnitude as well as position. Unlike the first mentioned patented system, the second patented system employs a tachometer generator mechanically coupled to the engine which outputs a waveform proportional to the position and speed of the rotor. A processor is employed to convert the fundamental vibration signal into its separate fundamental frequencies utilizing well known fast Fourier transform techniques. By determining the phase relationship of the vibration frequency component of interest to that of the generator signal, it is possible to determine the corrective weight magnitude and location when an imbalance condition exists.
In the above described balancing technique, the tachometer generator is indexed to the rotor so that as the rotor turns one revolution, the tachometer outputs a sinewave of one cycle. By physically indexing the tachometer generator output to an index/mark on the rotor, the position of the rotor is described by the output of the tachometer generator. Thus, the phase relationship between the tachometer generator output and the rotor vibration signal determines the physical location of the rotor unbalance.
Heretofore, the indexing of the rotor has been achieved by determining which fan blade of the rotor is aligned with the north pole of the windings of the tachometer generator. Typically, a hand magnet or compass is used and the procedure requires the removal of the primary exhaust plug which is held in place by approximately forty screws. The removal of these screws creates unnecessary rework when screws break due to crystallization caused by engine exhaust heat. Moreover, it has been determined that the tachometer generator windings embedded in the housing can be oriented differently from engine to engine thus creating errors during indexing of the rotor.